The present invention relates to a metal foil etched to roughen the surface (hereinafter sometimes referred to as xe2x80x9cetched foilxe2x80x9d) and a capacitor using the etched foil.
Capacitors for use in electronic instruments are demanded to have a small size and a large capacitance. As one of such capacitors, a capacitor using an etched aluminum foil for one part electrode is known. In this capacitor, the electrode is etched from the surface toward the inside to a predetermined thickness and thereby enlarged in the actual surface area. As the surface area is larger, the capacitance can be larger. Therefore, etching conditions, etching method, etching agent and the like have been studied with an attempt to enlarge the surface area. With respect to the etching method, a chemical etching method and an electrical etching method are usually combined to reduce the etching pore size or increase the etching depth and thereby enlarge the surface area. However, there is a limit in enlarging the surface area while keeping various strengths of the electrode. Studies are also made to more increase the thickness of the electrode than usual and thereby attain the enlargement of surface area and the maintenance of strengths, however, this contradicts the downsizing of capacitors and cannot be a substantial improvement.
On the other hand, as a capacitor having a small size and a large capacitance, a capacitor using a sintered body of tantalum for one part electrode is known. In this capacitor, a tantalum powder is molded and then sintered and the surface area of voids among powders inside the sintered body is utilized for the capacitance of capacitor. However, in the case of a relatively large sintered body, it is difficult to impregnate another part electrode (usually, a solid compound) into the vicinity of the center of the sintered body. As a result, the appearance capacitance is disadvantageously small for the surface area. Furthermore, the distance from the outer surface to the center of the sintered body is long and therefore, if the another part electrode is the same, the capacitor using a sintered body for the electrode is inferior in the high frequency performance as compared with the capacitor using the above-described aluminum etched foil for the electrode.
Despite these problems, in the capacitor using the tantalum sintered body for one part electrode, an oxide of tantalum can be used as a dielectric material and the dielectric constant of the tantalum oxide is about three times as large as the dielectric constant of the aluminum oxide. Therefore, tantalum is an attractive material for enlarging the capacitance and many studies have been made thereon so as to solve the above-described problems.
One of such studies is an attempt to manufacture an etched foil of tantalum as an electrode material. However, desired etching is impossible or difficult on the foil of an earth-acid metal itself including tantalum. For example, tantalum dissolves in a hydrofluoric acid and therefore, a method of electrolytically oxidizing tantalum in the hydrofluoric acid to manufacture an etched foil is employed. However, in practice, only the edge part of tantalum foil dissolves and etching of the entire foil surface is failed. Studies were once made to add additives to the starting material tantalum and thereby improve the etching property, however, this could not be a remarkable technical improvement. For example, in JP-A-46-7251 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d), a tantalum foil is obtained after adding a vanadium compound to tantalum or mixing a tantalum-vanadium alloy with tantalum, but the etching property is not improved.
Niobium belongs to the same group as tantalum. A niobium oxide is higher in the dielectric constant than the tantalum oxide and therefore, it is considered that by manufacturing an etched foil of niobium, a capacitor having a higher capacitance can be produced. However, this is not practiced up to the present.
As a result of extensive investigations to solve the above-described problems, the present inventors have found that even if desired etching is impossible or difficult on the foil of an earth-acid metal itself, an alloy foil comprising an earth-acid metal as a main component can be etched under specific conditions. The present invention has been accomplished based on this finding. The main component is a component contained in a largest proportion. A component contained in a proportion of 50% or more is a main component.
The reasons why the etching can be made are not clearly known but can be presumed because the bonding between atoms of an earth-acid metal itself becomes weak when alloyed, as a result, resistance against electro-chemical or chemical corrosion is deteriorated.
That is, the present invention relates to the following matters.
(1) A metal foil having a surface roughened by etching, wherein the metal is an alloy comprising an earth-acid metal as a main component.
(2) The metal foil as described in 1 above, wherein the earth-acid metal is niobium.
(3) The metal foil as described in 1 or 2 above, wherein the alloy comprising an earth-acid metal as a main component is an alloy with at least one element selected from the group consisting of Groups 3 to 16 by the group number of the periodic table.
(4) The metal foil as described in any one of 1 to 3 above, wherein the metal foil is partially nitrided.
(5) The metal foil as described in 4 above, wherein at least a part of the surface of the metal foil including the surface of inside pores is partially nitrided.
(6) The metal foil as described in any one of 1 to 5 above, wherein the metal foil has pores to a depth of at least 5% or more of the foil thickness in the foil thickness direction.
(7) The metal foil as described in any one of 1 to 6 above, wherein the metal foil has a surface area at least 50 times or more the surface area before etching.
(8) The metal foil as described in 6 above, wherein pores have an average pore size of 0.05 to 3 xcexcm.
(9) The metal foil as described in any one of 1 to 8 above, wherein the concentration of impurity elements other than tantalum, niobium, oxygen, nitrogen and an alloy-forming metal element is 300 mass ppm or less.
(10) The metal foil as described in 1 to 9 above, which gives an electrostatic capacitance of at least 200 xcexcF/cm2 or more per area (projected area) not including pore areas of the metal foil, to a capacitor fabricated from the metal foil as one part electrode, a dielectric material formed on the surface of the metal foil by electrolytic oxidation at 10 V, and another part electrode provided on the dielectric material.
(11) A capacitor comprising a pair of electrodes and a dielectric material interposed between the pair of electrodes, wherein the metal foil described in any one of 1 to 10 above is used for at least one electrode.
(12) The capacitor as described in 11 above, wherein the dielectric material comprises a tantalum oxide or a niobium oxide as a main component.
(13) The capacitor as described in 11 above, wherein the dielectric material comprises a ditantalum pentoxide or a diniobium pentoxide as a main component.
(14) The capacitor as described in 12 or 13 above, wherein the dielectric material is formed by an electrolytic oxidation.
(15) A capacitor comprising a pair of electrodes and a dielectric material interposed between the pair of electrodes, wherein one part electrode is the etched foil described in 1 to 10 above and another part electrode is at least one material selected from the group consisting of electrolytic solutions, organic semiconductors and inorganic semiconductors.
(16) The capacitor as described in 15 above, wherein the dielectric material comprises a tantalum oxide or a niobium oxide as a main component.
(17) The capacitor as described in 15 above, wherein the dielectric material comprises a ditantalum pentoxide or a diniobium pentoxide as a main component.
(18) The capacitor as described in 16 or 17 above, wherein the dielectric material is formed by an electrolytic oxidation.
(19) The capacitor as described in 15 to 18 above, wherein the another part electrode is composed of an organic semiconductor and the organic semiconductor is at least one organic semiconductor selected from the group consisting of an organic semiconductor comprising a benzopyrroline tetramer and chloranile, an organic semiconductor mainly comprising tetrathiotetracene, an organic semiconductor mainly comprising tetracyanoquino-dimethane, and an organic semiconductor mainly comprising an electrically conducting polymer obtained by doping a dopant into a polymer containing a repeating unit represented by the following formula (1) or (2): 
(wherein R1 to R4 each independently represents a monovalent group selected from the group consisting of a hydrogen atom, a linear or branched, saturated or unsaturated alkyl, alkoxy or alkyl ester group having from 1 to 10 carbon atoms, a halogen atom, a nitro group, a cyano group, a primary, secondary or tertiary amino group, a CF3 group, a phenyl group and a substituted phenyl group; the hydrocarbon chains of R1 and R2, or R3 and R4 may combine with each other at an arbitrary position to form a divalent chain for forming at least one 3-, 4-, 5-, 6- or 7-membered saturated or unsaturated hydrocarbon cyclic structure together with the carbon atoms substituted by R1 and R2 or by R3 and R4; the cyclic combined chain may contain a bond of carbonyl, ether, ester, amide, sulfide, sulfinyl, sulfonyl or imino at an arbitrary position; X represents an oxygen atom, a sulfur atom or a nitrogen atom; and R5 is present only when X is a nitrogen atom, and independently represents a hydrogen atom or a linear or branched, saturated or unsaturated alkyl group having from 1 to 10 carbon atoms).
(20) The capacitor as described in 19 above, wherein the organic semiconductor is at least one member selected from the group consisting of polypyrrole, polythiophene, polyaniline and substitution derivatives thereof.
(21) The capacitor as described in 19 above, wherein the electrically conducting polymer is an electrically conducting polymer containing a repeating unit represented by the following formula (3): 
(wherein R6 and R7 each independently represents a hydrogen atom, a linear or branched, saturated or unsaturated alkyl group having from 1 to 6 carbon atoms, or a substituent for forming at least one 5-, 6- or 7-membered saturated hydrocarbon cyclic structure containing two oxygen elements when the alkyl groups are combined with each other at an arbitrary position; and the cyclic structure includes a structure having a vinylene bond which may be substituted, and a phenylene structure which may be substituted).
(22) The capacitor as described in 21 above, wherein the electrically conducting polymer is an electrically conducting polymer obtained by doping a dopant into poly(3,4-ethylenedioxythiophene).